one phenomenon i see in the pro-audio business is speaker systems that sound dry and dead with high damping factor amps (1000 and above), but sound much better with a medium (100-900)damping factor, and some systems sound boomy and loose with medium damping factor, and sound great with a high damping factor. has anyone come up with a way to create a variable damping factor in an amp? i think i have, but i need to model it first, just to see if it's even do-able.

ok, i modeled this 2 different ways, first with simply an output stage to model the effect of changing the output emitter resistors, the second using a complete amp with 1 ohm emitter resistors to see the effect of feedback.

the first model returned the results i expected, a properly biased output stage with no feedback has an output impedance equal to all of the emitter resistances paralleled, plus all of the on resistances of the output devices paralleled, plus the power supply impedance.

the second model surprised me a bit.
i stepped the Av of the amp in a 1-2-5 series from 1 to 10000, expecting the output impedance to be inversely proportional to the feedback (all i changed was the feedback resistor, from 1k to 10Meg), but it wasn't. it started out at 7 or 8 milliohms, and stayed there until i got Av up to about 100. then the impedance began to change until it was about .5 ohms at Av=10000.

i'll post the actual results tomorrow (sorry, left the data at home.....) along with the schematics of the models.

i was thinking of having the damping factor set by jumper settings on the amp board, to minimize the possibility of problems due to OHS errors (Operator HeadSpace) or at least a variable internal control. the idea being to have the amp feedback variable, and have a preamp opamp with variable gain in an inverse ratio, so the overall gain from input to speaker remains the same. only problem is the amp distortion will also go up, so the open loop distortion has to be super-low to begin with. i can probably do that with a design that has a medium (in the mid thousands) open loop gain to begin with, as long as i can keep distortion to a minimum (less than 1% open loop). the other method would require rather large (physically) resistors and would add a small amount of power loss, but would definitely change the damping factor to one or more preset ratios.

Very simple. Take an amplifier with the highest damping factor desired, and simply start adding resistance until you get the lowest. An external series resistance will have exactly the same effect as any amplifier adjustments.

But as Sreten said, there is no way that damping factors above say 50 can have any audible effect. The basic laws of physics preclude that. If such effects were noticed the explanation was elsewhere. And again with all respect, I must ask about the conditions under which such was noticed (heard)/compared.

Just for clarity (although I am sure you know this), the damping factor as currently defined is misleading and nonsense as an amplifier characteristic. It is supposed to be a measure of the "braking power" exerted on a loudspeaker. That occurs because of back emf shorted out by an external short across the loudspeaker terminals. But the current thus generated flows in a circuit (circle), the resistance of which must be all-inclusive. That is, it includes the dc resistance of the loudspeaker, which is unavoidable. For an 8 ohm loudspeaker this is usually >say 5,5 ohm.

Thus, even if the amp output impedance is a dead short, as in a damping factor of a million, the actual damping factor will still only be 8/5,5 or 1,5. One can now see how rediculous the everyday definition of df is as a real factor influencing braking.

To humour the situation: A df of 30 would mean an amplifier internal impedance of 8/30 or 0,266667 ohm. That will give a "braking" df of 8/5.266667 or 1,3873. Take a df of 1000, which indicates amplifier internal impedance of 0,008 ohm. The braking df with that would be 8/5,5008 or 1,4543.

Thus the second figure would indicate an actual physical df inprovement of under 5%. And this is audible?

Apology for the lengthy epistle, but qualitive arguments would not illustrate the effect so vividly. It serves to indicate that such real differences are simply negligible. As said, not calling anybody a liar, but there are just two many other factors to merit a variable df.